Tandem releasable bridge plug system and method for setting such tandem releasable bridge plugs

ABSTRACT

A method of setting a tandem releasable bridge plug system in a casing includes assembling a tandem bridge plug string including a lower bridge plug on a lower, disconnectable connector, an upper bridge plug on an upper, disconnectable connector which both are initially disabled by a lock and unlockable from topsides, and running in the tandem plug string on a drill pipe string until the lower plug is at its setting target depth; setting and shutting the lower plug; disconnecting the upper plug from the lower plug; pulling up the upper plug to its upper setting target depth; enabling the upper plug by releasing the lock; setting and shutting the upper plug; disconnecting the drill pipe string from the upper plug.

FIELD OF THE INVENTION

The present invention is a tandem releasable bridge plug system and amethod for setting such tandem releasable bridge plugs in a casing.

BACKGROUND ART

Retrievable bridge plugs or “RBP's” are well known in the well serviceindustry. A retrievable bridge plug carried on a drill pipe string maybe set by the following actions:

-   -   using drag blocks to set up a rotational resistance,    -   rotating the string for setting slips,    -   rotating the string for setting a packer,    -   pressure testing the packer sealing ability from below by        pressurizing the drill pipe string,    -   rotating the string for closing a ball valve in the bridge plug,    -   rotating the string for disconnecting the drill pipe string from        the bridge plug,    -   pressure testing the closed bridge plug from above by        pressurizing the drill pipe string while the casing valve is        closed.

The drill pipe string rotational movements may be to the right or leftand corresponding axial movements being up or down may be specialdesigned. The deck crew will know the required steps for a particularconnector or “running tool” and perform them sequentially on the deck onthe drill pipe string “in blind”, by manipulating the drill pipe stringtopside. Such combinations of rotations and axial forces may be:

-   -   RH rotation and axial downward weight to set the slips, and    -   set the packer, (the drag blocks set up a counterforce for        against the drill pipe string rotation by their friction against        the casing wall)    -   subsequent LH rotation to close the ball valve,    -   downward force and LH rotation and upward pull to release the        connector, and    -   pull up to disconnect the drill pipe string connector and remove        it from the top of the retrievable bridge plug.

Other rotations and forces may be applicable and is subject to the tooldesigner's discretion.

An opposite sequence of the opposite steps may be used to retrieve thetool:

-   -   run in hole with the drill pipe sting with the connector,    -   down force and RH rotation to reconnect the drill pipe string        connector to the top of the retrievable bridge plug,    -   optionally shut the BOP to control possible gas release from        below plug,    -   subsequent RH rotation to open the ball valve,    -   upward force and RH rotation to release the packer, and    -   release the slips while the drag blocks still set up a        counterforce against the drill pipe string rotation by their        friction against the casing wall,    -   upward force to pull the retrievable bridge plug out of hole.        Such a sequence may be used for releasing an upper plug and        running it further in to release a lower plug and then        retrieving both on a common string. However, as will be        explained below, despite the fact that two such equally designed        plugs may be pulled in one run, they may not easily be set in        one run due to the manipulations on the drill pipe string will        work on the upper plug only, and then the lower plug remains        unset.

In the prior art U.S. Pat. No. 9,279,307, two consecutive plugs in awell are removed in one run. The plug of the prior art has no throughbore, and a special axially arranged release tool is required below theupper plug, and this is required before setting. However the US patentis not enabled to set two plugs in one run.

In the prior art U.S. Pat. No. 4,928,762 to Mamke, “Retrievable bridgeplug and packer”, he discloses a retrievable bridge plug and packerapparatus including a packer settable by right-hand rotation andvertical manipulation of the tool string, and a bridge plug alsosettable by right-hand rotation and vertical manipulation of the toolstring. A disabling tool is provided to disable the packer duringsetting of the bridge plug so that the packer is not prematurely set.After setting of the bridge plug below a formation, the packer anddisabling tool may be disengaged from the bridge plug. The disablingtool is disengaged so that the packer may be set above the formation.The packer may not be closed or disconnected from the drill pipe string.The packer is unset by pulling the tool string, and the packer anddisabling tool may be reconnected to the bridge plug.

U.S. Pat. No. 5,020,597 to Braddick et al., “Arrangement and method forconducting substance and lock therefor” describes a cementing string fora liner, with upper and lower wiper plugs with external seals thereonreleasably connected to the operating string. A lock arrangementprevents premature release of the upper and lower wiper by mechanicalforce, but is responsive to fluid pressure to first release the lowerwiper prior to release of the upper wiper.

US patent application 2004/0050546 relates to an arrangement ofsequentially configured packer J tools for one trip sequential settingof packer tools and for subsequent one trip sequential release of thetools.

U.S. Pat. No. 2,806,532 provides an improved method and apparatus forstraddling casing perforations and for applying high pressures safely tothe locations surrounding the casing perforations.

U.S. Pat. No. 4,794,989 discloses an apparatus for use in completing oilor gas wells having two or more perforated production zones whichincludes packers and assemblies for closing off the annular spacebetween the tubing string and the well bore intermediate adjacent zonesto isolate one from the other in order to produce from eachindividually.

US patent application 2004/0251024 describes a method for performingsingle trip perforation and packing operations via a downhole assemblyin a cased well bore. The assembly is provided with an upper packer anda lower packer and has fluid communication established therethrough. Theupper packer of the assembly is set to isolate a perforated productionzone by introducing pressurized fluid through the assembly and againstthe casing below the lower packer of the assembly.

A first problem in the industry, since the lower plug is set bymanipulating the drill pipe string through rotational left or right,and/or axial movements up or down, and if combining two similarlyoperated plugs, one may hardly control, upon manipulating the drill pipestring rotationally and axially, which plug does what. One solution maybe to design the plugs with mutually excluding different setting andreleasing mechanisms but this requires much special preparation of eachtool and a large stock of tools with separate setting and releasemechanisms. The result, in practice, is that one may only set one plugon one drill pipe string at a time, requiring a second run for settingthe second plug. So setting the plug may take twice the time compared toremoving them. Further, setting two plugs fast during an emergency isnot possible with the prior art.

A second problem in the industry is, since it is only possible to setone plug in one run, one may only have the time required to set one plugin a safety disconnect plugging operation such as for disconnecting theupper part of the drill pipe string and hanging off the major part ofthe drill string in one plug in the borehole.

A third problem is that in order to use two plugs for search andlocation of a casing string leakage, one may run a lower plug on a firstdrill pipe string and then locate the lower bound of the leak, then runan upper plug on a second drill pipe string to locate the upper bound ofthe leak, and thus locate the location of the leak. Such a leak may bein a casing string of continuous even diameter, or it may be in atransition from one casing diameter to a liner diameter, i.e. fortesting a liner hanger seal.

SHORT SUMMARY OF THE INVENTION

A main object of the present invention is to disclose a tandemreleasable bridge plug system and a method for setting such tandemreleasable bridge plugs. The method of the invention is as follows: amethod of setting a tandem releasable bridge plug system in a casing(0), comprising the steps of:

-   -   assembling a tandem bridge plug string (1L,1U) comprising from        bottom to top:        -   a lower bridge plug (1L) on a lower, disconnectable            connector (2L), and        -   an upper bridge plug (1U) on an upper, disconnectable            connector (2U) which upper bridge plug (1U) and connector            (2U) are initially disabled by a lock (5) and unlockable            from topsides, and    -   running in the tandem plug string (1L,1U) on a drill pipe string        (4) until the lower plug (1L) is at its lower plug setting        target depth (dL) in the casing (0);    -   setting the lower plug (1L); setting a packer (16L) of the lower        plug (1L); pressure integrity testing the sealing effect of the        lower plug's (1L) packer (16L) from below by pressurizing the        drill pipe string (4) topsides, and shutting the lower plug        (1L);    -   disconnecting the upper plug (1U) from the lower plug (1L);    -   pulling up the upper plug (1U) to its upper plug setting target        depth (dU) in the casing (0),    -   enabling the upper plug (1U) by releasing the lock (5);    -   setting the upper plug (1U) setting a packer (16U) of the upper        plug (1U); pressure integrity testing the sealing effect of the        upper plug's (1U) packer (16U) from below by pressurizing the        drill pipe string (4) topsides, and shutting the upper plug        (1U);    -   disconnecting the drill pipe string (4) from the upper plug        (1U).

The plug system used in the above method is a tandem releasable bridgeplug system arranged for setting a in a casing (0),

-   -   said tandem bridge plug string (1L,1U) comprising from bottom to        top:    -   a lower bridge plug (1L) on a lower, disconnectable connector        (2L), and    -   an upper bridge plug (1U) on an upper, disconnectable connector        (2U) which both are arranged for being initially disabled by a        lock (5) and unlockable from topsides, and    -   said tandem plug string (1L,1U) arranged for being run in on a        drill pipe string (4) until the lower plug (1L) to a lower plug        setting target depth (dL) in said casing (0);    -   said lower plug ((1L) arranged for being set in said casing (0)        and thereafter setting a packer (16) of said lower plug (1L);        said lower plug arranged for pressure integrity testing of its        packer's (16L) sealing effect from below by pressurizing said        drill pipe string (4) topsides, said lower plug (1L) arranged        for being shut,    -   said upper plug (1U) arranged for being disconnected from said        lower plug (1L);    -   said upper plug (1U) arranged for being enabled by releasing the        lock (5);    -   said upper plug (1U) arranged for being pulling up to an upper        setting target depth (dU) in the casing (0).

The first problem of setting two plugs in one run is thus solved by theinvention. This reduces the setting time for two plugs by about 50%.Time is costly both from an economical and a safety view; a drilling rigand particularly a marine drilling platform has high day rates. Time maycostly from a safety point of view if the two plugs are to set fast inorder to remedy an undesired state of the well. The invention hasseveral further significant advantages.

The second problem of not being able to set two plugs during a safetydisconnect operation is solved by the invention. First, if the lowerplug is set, optionally with a hang-off drill string below, then themost imperative problem is solved, the well is plugged by one plug, andone may leave the well in an emergency by disconnecting from the well.If time allows after setting the first, lower plug, one may furtherquickly set the upper plug as a backup to further reduce the risk ofblow out, and then disconnect and prepare to leave the well.

The third problem of not being able to conduct a single run pressuretesting for leak location in a casing string is solved.

FIGURE CAPTIONS

The attached figures illustrate some embodiments of the claimedinvention.

FIG. 1A to FIG. 1I illustrate embodiments of the invention's method; thesteps of setting two plugs in a cased well.

FIG. 1A shows run in hole for the assembled upper plug (1U) and lowerplug (1L) until the latter is at its lower plug setting target depth(dL). In an embodiment both plugs are through-bore. In a preferredembodiment both plugs are provided with an axial bore ball valve. In apreferred embodiment initially their main bore valves are open.

FIG. 1B illustrates that weight and right hand (RH) rotation is exertedon the drill pipe string from topside, the lower plug is then set, butnot necessarily closed. “Set” here may imply setting the slips andsetting the expandable packer. Optionally pressure P may be set on thedrill pipe string from topside to test the still open lower plug (1L)from below pressure.

FIG. 1C illustrates left hand (LH) rotation to close the valve of theset lower plug (1L).

FIG. 1D shows a further step of combined push and subsequent left-handrotation (LH) combined with pull on the drill pipe string (4) to releasethe lower connector (2L) from the top of the lower plug; please also seeFIG. 4. FIG. 1D shows pulling the upper plug (1U) to its upper plugtarget depth (dU). Now with the lower plug (1L) closed and with orwithout an intermediate drill pipe string (3), open at its lower end,and with the through bore connector (2L) open, one may optionally setpressure P on the drill pipe string (4) from topside to test the nowshut lower plug (1L) using above pressure from above. The above pressuretest procedure is usually satisfactory on the first run, but if thepressure tests are not satisfactory, with the upper plug (1U) lock (5)disabling the upper plug (1U) from being set, it is still possible toopen, release and reposition the lower plug (1L), and test again,because the upper plug (1U) is still un-enabled; it only works inprinciple as any drill pipe string section as part of the drill pipestring (4) and intermediate drill pipe string (3) all extending fromtopside, except for its upper drag blocks (12U) dragging along thecasing inner wall (0i).

FIG. 1E illustrates a subsequent key step of the invention, wherein aball or dart is dropped to unlock and enable the upper plug (1U). In anembodiment of the invention, it unlocks the lock (5) to enable the upperconnector (2U) and enables the upper plug (1U) so as for enabling themto make the rotational and axial movements required to set and close theupper plug (1U) and to disconnect the upper plug (1U); please see thesubsequent series of illustrated steps. From now on, the upper plug (1U)is operable from topside and in an embodiment it may be of the same typeas the lower plug (1L), albeit not necessarily of the same dimension.

FIG. 1F shows setting the now enabled upper plug (1U), preferably bysetting its slips (14U) and its expandable packer (16U), both by weightdown and right hand rotation, and an optional step of pressure testingthe set packer (16U) of the upper plug (1U) from below.

FIG. 1G illustrates a subsequent step of closing the upper plug (1U) byleft-hand (LH) rotation. Please notice that because one has disconnectedthe upper plug (1U) from the lower plug (1L), any rotational and axialmovements for operating the upper plug (1U) does not affect the lowerplug (1L) which is set, simply.

FIG. 1H illustrates a next step of pulling upward and left hand rotation(LH) in order to disconnect the upper connector (2U) from the upper plug(1U). Optionally one may now pressure test from above. It is nowpossible to reconnect, open, release, and reposition the upper plug (1U)if the pressure test is not satisfactory.

FIG. 1I shows that the two plugs are now set in their plug settingtarget depths and from the topside one may pull up (PU) or pull out ofhole (POOH) with the drill pipe string (4) and the upper connector (2U).

FIG. 2A to FIG. 2D illustrate an embodiment of the upper plug (1U) in alongitudinal section, the proximal end to the left.

FIG. 2A shows, in a run in hole state, a longitudinal section of theupper plug (1U) with the disconnectable connector (2U) to the left,having a lock unit (5) for temporarily inactivating the connector (2U);a section with a central bore upper valve (18U); a section with theupper plug's packer and slips (16U, 14U) and with upper pluug's dragblocks (12U). A ball (51) is shown being dropped and about to land in aball seat of a ball seat axial sleeve (52), thus enabling a heredisabled collet sleeve (53) which has inward yielding collet splines, tobe axially moveable under axial force within the upper connector sleeve(2U) when released. The ball valve (18U) is not closed, thus the throughcentral bore (19) of the upper plug (1U) is open. The slips (14U) arenot yet extended. The packer (16U) is not yet set. The drag blocks (12U)are extended and will cause friction drag on the inner casing wall (0i)if the tool is moved axially.

FIG. 2B shows the ball (51) having released the ball seat axial sleeve(52) which has passed through the tool (1U) and has sheared out andreleased the second shear seat (55) which so far had held theanti-rotation pins (55P) which so far have disabled independent corerotation of the setting mechanism for the RH rotation of the slips (14U)and the corresponding compression of the packer (16U). Please see theball (51), the ball seat axial sleeve (52) and the second shear seat(55) now caught in the ball catcher to the right.

FIG. 2C illustrates the further step of setting the plug and unlockingthe ball valve by screw-compressing the section with packer and slips(14U, 16U), the stem no longer prevented from relative rotation by theanti-rotation pins (55P), and is able to rotate relative to the barrelheld still relative to the inner casing wall (0i) by the drag blocks(12U). The rotation of the stem engages a screw mechanism to extend theslips (14U) and set the packer (16U). The collet sleeve (53) may now bepushed in axially, upwardly, by stinger (2U1) on top of the upper plug(1U).

FIG. 2D shows the further step wherein the upper plug (1U) has been setand the ball valve (18U) is closed, preferably comprising shutting theball valve (18U) by LH rotation, thus closing the central bore (19) ofthe upper tool (1U).

FIG. 3A is a lateral view of the upper connector sleeve (2U).

FIG. 3B is a longitudinal section view of the upper connector sleeve(2U), i.e. the sleeve portion of the connector (2U) which shall combinewith the upper plug's (1U) top stinger portion (2U1). Please see FIG.2A, 2B, 2C for their operations in sequence. In the central bore (19) ofthe connector sleeve (2U) there is arranged the above mentioned colletsleeve (53) temporarily blocked by the ball seat sleeve (52) of the lock(5) for temporarily inactivating the connector (2U). The connector (2U)is enabled by dropping a ball (51) down through the drill pipe string(4). Please see below for ball seat lock details. Further FIG. 3Bdepicts an internal shoulder (2Ui) for knobs on inward yielding colletsleeve (53) to be held back before sheared out seat sleeve (52) isremoved, thus preventing the upper stinger (2U1) to move upwardly, thusenabling the J-slot mechanism of the upper connector. The upperconnector (2U) has J-slots (2U3) for dogs (2U2) on the upper stinger(2U1). In the shown embodiment the J-slots are arranged for push andleft hand rotation of the drill string to release.

FIG. 4 shows the lower connector sleeve (2L) in a longitudinal sectionview. It shows an embodiment of the invention wherein the lowerconnector (2L) is arranged for being subject to weight, then a lowerconnector collet (2Lc) collapses by yielding outwardly and allows thatthe running and retrieval tool (the lower connector 2L) with itsinternal J-slots may be pushed downward on the lower plug's (1L) stingerportion (2L1) with radial dogs (2L2), so as for the radial dogs (2L2) tobe guided upwardly in the J-slot (2L3), then we rotate to the left (LH)to release, and pull up. After these operations, the lower connector(2L) is free from the lower plug's (1L) stinger portion (2L1) at itstop.

The Invention and Embodiments of the Invention

The invention will in the following be described and embodiments of theinvention will be explained with reference to the accompanying drawings.

Main Invention:

The invention is a method of setting a tandem releasable bridge plugsystem in a casing (0). The method comprises, in its minimum, broadestdefinition, the steps of:

-   -   assembling a tandem bridge plug string (1L,1U) comprising from        bottom to top the following elements:        -   a lower bridge plug (1L) on a lower disconnectable connector            (2L);        -   an upper bridge plug (1U) on an upper disconnectable            connector (2U) which both are initially disabled by a lock            (5) and unlockable from topsides;    -   running in the tandem plug string (1L,1U) on a drill pipe string        (4) until the lower plug (1L) is at its plug setting target        depth (dL) in the casing (0), please see FIG. 1A,    -   setting and shutting the lower plug (1L),    -   disconnecting the upper plug (1U) from the lower plug (1L),    -   pulling up the upper plug (1U) to its upper plug setting target        depth (dU) in the casing (0),    -   enabling the upper connector (2U) and the upper plug (1U) by        releasing the lock (5);    -   setting and shutting the upper plug (1U);    -   disconnecting the drill pipe string (4) from the upper plug        (1U).

After the above steps one may pull out the drill pipe string (4) fromthe well or not, depending on the subsequent operation to be made on thewell.

The disconnectable connectors (2U, 2L) are both usually named “runningand retrieving tool” of which an upper half, the upper sleeve portions,please see FIGS. 3A, 3B, 4 belongs to a lower end of a running drillpipe string, here the drill pipe string (4) and the intermediate drillpipe string (3), and the corresponding lower half, a stinger portion(2U1, 2L1) with dogs for fitting into corresponding J-slots (2U3, 2L3)respectively, which belong to the upper retrievable bridge plug (1U) andthe lower retrievable plug (1L), respectively. For the upperdisconnectable connector (2U) please see FIG. 2A in its initial, lockedand disabled state.

Lower Bridge Plug:

The lower bridge plug (1L) is set according to the above described stepsunder the background art. A problem is how to set the lower bridge plug(1L) without setting or releasing the similarly operated upper bridgeplug (1U), then release the upper bridge plug (1U) from the lower bridgeplug (1L), and first now enabling the mechanism for initiating andconducting setting and release of the upper bridge plug (1U). In thisway the tandem plugs (1L, 1U) may be set on one and the same run.

Lower Plug Repositioning Possible:

Before the upper plug (1U) is enabled, the lower plug (1L) may berepositioned until its operational required slip holding force andsealing property against the inner casing wall (0 i) is met. After theupper plug (1U) is enabled, the lower plug (1L) may thereafter inpractice only be reconnected, released and retrieved, preferablytogether with the upper plug (1U). We therefore do not show the releasesequence because it is understandable as an inverse sequence when thesetting sequence herein is explained.

Initially disabled upper plug:

The initially disabled upper bridge plug (1U) and its disconnectableconnector (2U) are, before and during run-in and setting of the lowerbridge plug (1L), temporarily disabled from being settable by anyrotational and axial movements by the lock (5). In effect, as seen fromthe lower plug's (1L) side, except for the drag blocks (12U) of theupper bridge plug (1U), the upper plug (1U) just forms a passive part ofthe drill pipe string (4). Thus the deck crew may control the lowerbridge plug (1L) as if it were the only bridge plug on the drill pipestring (4) until the lower bridge plug (1L) is set, disconnected, andthe upper bridge plug (1U) is enabled.

Ball Seat Lock Details:

In an embodiment of the invention, the initial disabling of the upperbridge plug (1U) is made by a ball seat axial sleeve (52) arranged in acollet sleeve (53) constituting the lock (5) of the connector (2U),please see FIG. 3a . The ball seat axial sleeve (52) prevents thecollets of the collet sleeve (53) with its external dogs from yieldinginwardly, thus hanging on a rim shoulder preventing the collet sleeve(53) to move upwardly. The ball seat axial sleeve (52) is releasable bya drop ball (51) and will pass downwardly through the central bore (19)of the plug, to allow the stinger (2U1) backed collet sleeve (53), withits external dogs on collet sleeves, be forced upwardly to unlock thelock (5), thereby enabling the release tool (2U) to be collapsed on itscorresponding stinger portion (2U1) which is mounted on top of the upperplug (1U), please also see FIG. 3b . The force required to move thecollet sleeve (53) when enabled, is in an embodiment 6½ to 7½ metrictonnes (MT) weight exerted by the drill pipe string (4), while othervalues may be selected by another mechanical designer skilled in theart.

Ball Catcher:

In an embodiment of the invention the ball (51) and the ball seat axialsleeve, hereafter called the first ball sleeve (52), may be transportedby the pressure through the central bore and eventually be caught in aball catcher (31) at the lower end of the upper retrievable bridge plug(1U). It is undesirable that the ball and sleeve shall drop freely intothe casing below the upper plug (1U) as it would interfere with thelower plug (1L) on attempting reconnecting for retrieval and also createother problems. The relative shear pressure of the first seat sleeve(52) is in an embodiment set to 69 Bar (1000 psi).

When the release tool section (2U) lock mechanism is enabled tocollapse, and the connector's J-slot mechanism to be enabled to preparefor contract, rotate in a given direction to release, an upper plug (1U)initiation and setting sequence may be activated.

Second “Ball” Shear Seat, Anti-Rotation Pins:

In an embodiment of the invention the released down going seat sleeve(52) will bring along with it a second seat sleeve (55), please seebetween the upper plug's (1U) drag block unit (12U) and the slip unit(14U) in FIG. 2A, the second seat sleeve (55) hitherto disabling therotational movement for engaging the slips (14U) and the packer (16U).This second seat sleeve (55) has shear pins requiring a lower pressureto release than the first seat sleeve (52), so if the first sleeve (52)is sheared, one will be sure that the second sleeve (55) is also shearedimmediately after. The second seat sleeve (55) releases upon its removaloppositely directed spring loaded anti-rotation pins (55P) radiallyinwardly, so as for allowing free rotation of the core stem relative tothe surrounding sleeve forming part of the screw setting mechanism forthe upper slips (14U) and packer (16U), thus enabling the rotationalmovement for the slip and packer activation to start. The relativepressure required for the second seat sleeve (55) is 54 Bar (780 psi) inan embodiment. Other shear pressures for both seat sleeves areselectable according to the discretion of a tool designer.

In an embodiment of the invention, after enabling the upper plug's (1U)release tool (2U), it is further enabled for the drill string (4) withthe release tool (2U), please see FIG. 1b , to operate on and rotate thedrill pipe string (4) relative to the drag blocks (12U) of the upperplug (1U) to engage its slips (14U) with the casing (0) and subsequentlyengaging the packer (16U) to seal against the casing (0), please seealso FIG. 1C and FIG. 1D, and FIG. 2C.

Intermediate Drill Pipe String:

In the above, the lower and upper plugs (2L, 2U) are connected and areseparated by any distance, at minimum only separated by the lowerconnector (2L), preferably also with the ball catcher. In an embodimentof the invention, it is advantageous to use an intermediate drill pipestring (3) below the upper plug (1U) and above the lower connector (2L)connected to the lower plug (1L). The length of the intermediate drillpipe string (3) should only be limited to an embodiment of the inventionwherein it corresponds to have a length slightly less than the distancebetween the lower plug setting target depth (dL) for the lower plug (1L)and the upper plug setting target depth (dU) for the upper plug (2U).This in order for allowing for disconnecting from the lower plug (1L)before placing and setting the upper plug (1U). If they were stillmechanically connected one could not rotate and move the upper drillpipe string (4) from topsides without affecting the lower plug (10. Asignificant advantage of having an intermediate string (3) length ofalmost the depth difference between lower and upper plugs' (1L, 1U9setting target depths (dL, dU) becomes evident if the target depthdifference is large: Given a lower target depth (dL) of, say 5000 m, andupper target depth (dU) of 1000 m, one would then have to run in lowerplug (1L), then 4000 m of intermediate string (3), then upper plug (1U),and 1000 m of drill pipe string (4), in order to reach lower targetdepth. It would then be an operational advantage that when lower plug(1L) is set, the lower disconnectable connector (2L) is released, thenthe upper plug (1U) is near below its upper target depth (dU) and israpidly set upon pulling out a short distance, and then, after testingfor pressure integrity, one may pull out the relatively short, here 1000m of drill pipe string (4) only. Thus the two purely setting relatedoperations are conducted consecutively. Those are embodiments of theinvention and may be varied with respect to relative lengths.

Free Upper Plug:

In an embodiment of the invention, the upper plug (1U) with its belowmounted intermediate drill pipe string (3) is released from the lowerplug (1L), and the upper plug (1U) may be enabled, not earlier. Afterthe upper plug is enabled and also free from the lower plug (1L), andany drill pipe string manipulation with rotation or axial movement, willnot affect the lower plug. The upper plug may now be set and pressuretested, and if necessary, released and repositioned for further testinguntil satisfactory, and then disconnected.

Ball Catcher:

In an embodiment of the invention, there is arranged a ball and seatcatcher (31) below the upper plug (1U) and above the lower connector(2L). The ball and seat catcher is provided with lateral bypass channelsfor maintaining fluid communication through to and including the lowerconnector (2L).

Testing Lower Plug from Below:

In an embodiment of the invention, after setting, and before shuttingthe lower plug by shutting the ball valve (18L) of the lower plug (1U),one may set the lower packer (16L), and then conduct pressure integritytesting of the sealing effect of the lower plug's (1L) packer (16L) frombelow by pressurizing the drill pipe string (4) topsides. If thepressure integrity test is not OK, resetting the lower plug (1L) isfeasible and a new test may be run until the pressure integrity testfrom below is OK.

Testing Lower Plug from Above:

In a further embodiment of the above, after shutting the lower plug (1L)and having disconnected the lower connector (2L) from the lower plug(1L), one may conduct pressure integrity testing the lower plug (1L)from above by setting pressure through the drill pipe string (4).

Testing Upper Plug from Below:

In an embodiment of the invention, one may test the upper plug frombelow by the following steps: after setting, before shutting the upperplug (1U), setting its packer (16U), and then conduct pressure integritytesting for verifying the sealing effect of the upper plug's (1U) packer(16U) from the casing space below the upper plug (1U), but above thelower plug (1L), by pressurizing the drill pipe string (4) topsides.This pressure test should preferably not be run simultaneously with the“from above” pressure test of the lower plug (1L) described above,because, in case of a failed test, one would not know which of the lowerand upper packers (16L, 16U) having a leakage.

Testing Upper Plug from Dbove:

Similar to above, in a further embodiment of the invention, one may testthe pressure integrity of the upper plug (1U) from above by, aftershutting the upper plug (1U) and disconnecting the drill pipe string(4), pressure integrity testing the upper plug (1U) from above bysetting pressure through the drill pipe string (4) and the upperconnector (2U).

The lower plug (1L) is, except for the absence of the lock mechanism (5)with its ball seat sleeve (52), the spline sleeve (53) and the secondshear seat (55), and the ball catcher (31), of similar construction tothe upper plug (1U) with regard to drag blocks (12L), slips (14L), andpacker (16L).

If the casing to be plugged is of even diameter the upper and lowerplugs (1U, 1L) are of the same diameter. In an embodiment the lower plug(1L) may be of a lower diameter than the upper plug (1U), e.g. if thelower plug (1L) shall be set in a liner below a liner hanger in a casing(0).

1.-13. (canceled)
 14. A method of setting a tandem releasable bridgeplug system in a casing, comprising the steps of: assembling a tandembridge plug string comprising from bottom to top: a lower bridge plug ona lower, disconnectable connector; an upper bridge plug on an upper,disconnectable connector, said upper bridge plug and upperdisconnectable connector initially disabled by a lock and unlockablefrom topsides, wherein disabling the upper bridge plug is initially madeby a ball seat axial sleeve arranged in a collet sleeve constituting thelock of the connector, the ball seat axial sleeve being releasable by adrop ball to allow the collet sleeve to be forced upwardly and unlockthe lock enabling the upper connector to be axially and rotationallyoperable relative to the upper bridge plug by the drill pipe string;running in the tandem plug string on a drill pipe string until the lowerplug is at a setting target depth thereof in the casing; setting thelower plug; setting a packer of the lower plug; pressure integritytesting the sealing effect of a packer of the lower plug from below bypressurizing the drill pipe string topsides, and shutting the lowerplug; disconnecting the upper plug from the lower plug; pulling up theupper plug to an upper setting target depth thereof in the casing;enabling the upper plug by releasing the lock; setting the upper plug;setting a packer of the upper plug; pressure integrity testing thesealing effect of the packer of the upper plug from below bypressurizing the drill pipe string topsides, and shutting the upperplug; and disconnecting the drill pipe string from the upper plug. 15.The method of claim 14, wherein said releasable ball seat sleeve isarranged for, after having been released by said drop ball, passingthrough said upper plug and shearing out and releasing a second shearseat and bringing along with the second shear sheet said second shearseat sleeve down to a ball catcher, said second shear seat initiallyholding anti-rotation pins disabling the rotational movement of thedrill pipe string to engage upper slips and said upper packer, whereinafter enabling the upper connector and the upper plug, using the upperconnector to operate on said upper plug and rotating the drill pipestring relative to drag blocks of said upper plug to engage said upperslips with the casing and subsequently engaging said upper packer toseal against said casing.
 16. The method of claim 14, further comprisingusing an intermediate drill pipe string below the upper plug and abovethe lower connector on the lower plug.
 17. The method of claim 14,further comprising using a ball and seat catcher below the upper plugand above the lower connector.
 18. The method of claim 14, furthercomprising, after shutting the lower plug and disconnecting from thelower plug, pressure integrity testing the lower plug from above. 19.The method of claim 14, further comprising, after shutting the upperplug and disconnecting the drill pipe string, pressure integrity testingthe upper plug from above.
 20. The method of claim 14, wherein the stepof setting the lower plug comprises, while allowing a lower set of dragblocks of said lower plug to drag on said casing's inner wall,activating a lower set of slips of said lower plug to engage and hold onan inner wall of said casing, said method further comprising, aftertesting said lower plug for pressure integrity from below, shutting saidlower plug by closing a lower ball valve to shut a central bore of saidlower plug.
 21. The method of claim 14, wherein the step of setting saidupper plug comprises, while allowing an upper set of drag blocks of saidupper plug to drag on an inner wall of said casing, activating an upperset of slips of said upper plug to engage and hold on said casing'sinner wall, said method further comprising, after testing said upperplug for pressure integrity from below, shutting said upper plug byclosing an upper ball valve in said upper plug to shut the central boreof said upper plug.
 22. The method of claim 20, the setting of saidlower plug comprising, while allowing said lower set of drag blocks ofsaid lower plug to hold on the inner wall of said casing, using righthand rotation and axial weight for activating said lower set of slips toengage and hold on said casing inner wall, and for activating said lowerpacker to seal against the inner wall of said casing.
 23. The method ofclaim 22, further comprising, after engaging said slips and setting saidpackers, using left hand rotation for activating a lower ball valve toshut.
 24. The method of claim 15, the step of setting the upper plugcomprising, after dropping said ball through the drill pipe string toenable axial and rotational movement of said upper connector, whileallowing said upper set of drag blocks to drag on an inner wall of saidcasing, manipulating said drill pipe string topsides using right handrotation and axial weight for activating of said upper set of slips toengage and hold on the inner wall of said casing, and activating of saidupper packer to seal against the inner wall of said casing.
 25. Themethod claim 24, further comprising, after dropping said ball, settingsaid drag blocks, activating said slips and setting said packer, andusing left hand rotation for activating an upper ball valve to shut forclosing a central bore of said upper plug.
 26. A tandem releasablebridge plug system arranged for setting in a casing, with a tandembridge plug string comprising from bottom to top: a lower bridge plug ona lower, disconnectable connector and an upper bridge plug on an upper,disconnectable connector, said tandem bridge plug string wherein: saidupper bridge plug and said upper, disconnectable connector are arrangedfor being initially disabled by a lock and unlockable from topsides,said lock comprising a ball seat axial sleeve arranged in a colletsleeve, the ball seat axial sleeve being releasable by a drop ball toallow the collet sleeve to be forced upwardly and unlock the lockenabling the upper connector to be axially and rotationally operablerelative to the upper bridge plug by a drill pipe string; said tandembridge plug string arranged for being run in on a drill pipe string withsaid lower plug to a setting target depth in said casing; said lowerplug is arranged for being set in said casing, a lower packer of saidlower plug for being set in said casing, said lower plug having athrough central bore with a lower ball valve so as for allowing pressureintegrity testing of a sealing effect of said lower packer from below bypressurizing said drill pipe string topsides, and closing said lowerball valve thus shutting said lower plug; said lower connector isarranged for disconnecting said lower plug from said upper plug; saidupper plug is arranged for being pulled up to an upper setting targetdepth in said casing; said upper plug is arranged for being enabled byreleasing said lock; said upper plug has an upper packer, said upperpacker arranged for being set, said upper plug having a through centralbore, with an upper ball valve so as for allowing pressure integritytesting of the sealing effect of the upper packer from below bypressurizing the drill pipe string topsides, said upper plug arrangedfor being shut by closing said upper ball valve; and said upperconnector is arranged for disconnecting said upper plug from said drillpipe string.
 27. The method of claim 15, further comprising using anintermediate drill pipe string below the upper plug and above the lowerconnector on the lower plug.
 28. The method of claim 15, furthercomprising using a ball and seat catcher below the upper plug and abovethe lower connector.
 29. The method of claim 16, further comprisingusing a ball and seat catcher below the upper plug and above the lowerconnector.
 30. The method of claim 15, wherein the step of setting thelower plug comprises, while allowing a lower set of drag blocks of saidlower plug to drag on an inner wall of said casing, activating a lowerset of slips of said lower plug to engage and hold on the inner wall ofsaid casing, said method further comprising, after testing said lowerplug for pressure integrity from below, shutting said lower plug byclosing a lower ball valve to shut a central bore of said lower plug.31. The method of claim 16, wherein the step of setting the lower plugcomprises, while allowing a lower set of drag blocks of said lower plugto drag on an inner wall of said casing, activating a lower set of slipsof said lower plug to engage and hold on the inner wall of said casing,said method further comprising, after testing said lower plug forpressure integrity from below, shutting said lower plug by closing alower ball valve to shut a central bore of said lower plug.
 32. Themethod of claim 17, wherein the step of setting the lower plugcomprises, while allowing a lower set of drag blocks of said lower plugto drag on an inner wall of said casing, activating a lower set of slipsof said lower plug to engage and hold on the inner wall of said casing,said method further comprising, after testing said lower plug forpressure integrity from below, shutting said lower plug by closing alower ball valve to shut a central bore of said lower plug.
 33. Themethod of claim 18, wherein the step of setting the lower plugcomprises, while allowing a lower set of drag blocks of said lower plugto drag on an inner wall of said casing, activating a lower set of slipsof said lower plug to engage and hold on the inner wall of said casing,said method further comprising, after testing said lower plug forpressure integrity from below, shutting said lower plug by closing alower ball valve to shut a central bore of said lower plug.